A pressure regulating valve is already known (German Offenlegungsschrift No. 23 29 667), in which to perform the closing function the regulating piston has a diameter reduced by comparison with that of the slide bore in such a way that pressure in a piston chamber can be relieved during the closing operation via the annular gap between the regulating piston and the slide bore. This relatively large amount of piston play, however, means that during the regulating operation as well, the pump has to pump a certain amount of fuel that flows uselessly back via the piston play, in the form of leakage. Such losses due to leakage make the prior art device relatively inefficient.